In order to reduce CO2 emissions from thermal power plants comprised of boilers and steam turbines, efforts have been continued to improve the efficiency of the thermal power plants by increasing the pressure and temperature as steam conditions. Nowadays, in Japan, mainly coal fired power plants of the 0.6-to-1 million kW class employ the following steam conditions for their commercial systems: a main steam pressure range from 24.1 to 25.0 MPa (supercritical pressure), a main steam temperature range from 593 to 600° C., a reheated steam temperature range from 593 to 620° C. (for example, see Non-patent Literature 1, “HITACHI HYORON”, Vol. 80, No. 2, published in February 1998, pp. 61-66). Generally, steam pressures of 24.1 MPa (3500 psi) or more and steam temperatures of 593° C. (1,100° F.) or more are called ultra-supercritical (USC) pressure conditions. The commercialization of high-temperature materials for boiler tubes, valves and turbines which are excellent in high-temperature strength and corrosion resistance has contributed largely to the practical use of such ultra-supercritical pressure steam conditions (steam temperatures of 593° C. or more).
On the other hand, small-to-medium capacity thermal power plants in which the power output of a single generator is 0.4 million kW or less have employed steam conditions that the main steam pressure remains subcritical and the maximum turbine inlet steam temperature is 566° C.